Analysing Chemical Reactions in Combining Two Particles

Introduction

The process of joining two particles (reactants), in order to form new product/products is called a reaction. The pace of a specific thing can be measured using the rate i.e., it describes how slow or fast a thing is. Rate is a measurement of the change that occurs in single unit of time, and its appropriate measurements are seconds, minutes, hours or even days. The pace with which the reactants react, is termed as the rate of the chemical reaction. Usually, for finding the rate of a reaction, either the amount of reactant used per unit of time or the amount of product produced per unit of time should be calculated.

During the process of reaction, the rate does not remain constant and it varies throughout the process. It is higher in the initial stages and it gets reduced as the reactions progresses. In my view, the longer the process of reaction, the higher the reactions from the more reactants ,Ieaving the fewer reactants with less chance of reacting

My investigation mainly deals with a factor, which affects the rate of the reaction. The collision theory ' explains the rate of reaction . The fundamental idea behind this theory is that the collision among the particles has to be hard enough for the reaction to take place. The rate of reaction mainly depends on how hard the reacting particles collide with each other.

Collision theory

When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. This can be termed as the "Collision Theory". The two molecules will react only if they have sufficient energy. Heating the mixture, will elevate the energy levels of the molecules involved in the reaction. Rising temperature denotes the faster movement of the molecules. This is termed as the "Kinetic theory".

Factors which affects the rate of reaction

A catalyst changes the rate of a chemical reaction but it remains chemically unchanged throughout the process. It speeds up the chemical reactions. A catalyst works by providing the reacting particles a surface to stick on, from where they can bump into each other. This increases the number of successful collisions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction. This is true as the reaction proceeds in a different pathway in the presence of a catalyst . An extra catalyst will absolutely make no difference.

We could also change the size of the solid particle/surface area to increase collisions by breaking it into smaller pieces. This implies the particles around it, in the solution will have more area to work on so, there will be more useful collisions. This produces a quicker reaction. If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So, the larger the surface area of the solid, the faster the reaction will be. Smaller particles have a bigger surface area than larger particle for the same mass of solid. For instance, chewing the food increases the surface area and quickens the digestion .

The concentration or pressure increases the number of collisions because if the solution is more concentrated, it implies there are more particles of reactants knocking between the water molecules, which make collisions between the significant particles more likely. For gases, by increasing the pressure, you squeeze the molecules together and this will enhance the frequency of collisions between them. This is also described as the "Collision Theory".

Activation energy is the amount of energy required for the reactants to successfully react. The reactants need a certain amount of energy, and any increase in extra energy, results in the successful collision.

My Experiment

I have chosen to investigate, how the concentration of reactants affect the rate of reaction because it would be easier to measure and control the concentration variable. Temperature would have been harder to control because it is hard to measure it to a specific point and keep it at that temperature because it's easily affected by the environment. Surface area would have been harder to investigate because the chips were in varied sizes and shapes. This made the measurement of the surface area difficult ,although if they were in a precise shapes or sizes like cubes, this would have been possible. I found it hard to think of a suitable catalyst to add in this reaction so, changing the rate of reaction by adding a catalyst would have been hard for me. It was far easier to investigate how concentration affects the rate of reaction.

I will carry out this experiment by altering the concentration of hydrochloric acid added to a given amount of powdered marble chip and measure the time it takes to give off a certain amount of gas.

The equation for this reaction is:

CaCO3(s) + 2HCl(aq) ==> CaCl2(aq) + H2O(l) + CO2(g)

Apparatus

Gas syringe

Burette

Conical flask

Glass Delivery tube

Beaker

Funnel

Distilled water

Top Pan Balance

Stopwatch

Hydrochloric Acid (1 Mole)

Ground Marble chips

Thermometer

Clamp

Bung

Prediction

My hypothesis describes, the higher the concentration of hydrochloric acid, the faster the carbon dioxide will be given off. This is because as the concentration of the acid is increased, there are a greater number of H+ ions (produced by the acid) in the same volume. Hence, there is a greater chance of the H+ ions colliding and therefore reacting with the atoms on the surface of the marble chips. Here are some diagrams explaining this phenomenon:

In the first diagram, there are a relatively small number of particles and therefore the chance that they collide will be fairly low. They will collide less frequently. In the second diagram, there are about twice as many particles in the same volume. Therefore, the particles will collide a lot more often. I also predict that the gradient of the graph when I compare al the results will be a curve, starting low, and then steeping very high at the end:

Method

Clamp gas syringe level with a conical flask and bung on a table

Weigh 6g of ground marble chips using a top pan balance and place it inside the conical flask

Use a burette to measure the volume of HCL and another to measure the volume of water. Decrease the quantity by 10% each time (200ml of reactants)

Place both in a beaker, then pour it into the conical flak

Ask a partner to start the stop watch while you put the bung in place

Record how long it takes for the volume of gas to reach 50ml

Repeat experiment 3 times

Calculate the average it takes for the volume of gas to reach 50ml

Plot a graph to show this

To keep my investigation consistent and make sure there are no other factors ,which could affect the rate of reaction, I will be using the same equipments and same amount of grounded marble chips in each experiment.

I will use ground marble chips because it would give me a more accurate result than marble chips. This is because, the surface area of the marble chips would roughly be about the same where as larger pieces of marble chips would have different amount of surface area. This would affect the rate of reaction because there will be different amount of surface area for the reactants to work on. By making the surface area roughly the same, the rate of reaction won't be affected by it.

From my preliminary results, I conclude I would use 3g of marble chips because after each reaction, there were some residual marble chips . If there was a shortage of marble chips and the hydrochloric acid wasn't used up, the volume of gas wouldn't be able to reach 50ml so, in order to achieve this, we need to measure the amount of marble chips to be able to compare the results. We also need to measure the amount of marble chips to make the results more accurate. The quantity of marble chips would increase the rate of reaction because there would be more marble chips to collide with the acid. To make the experiment less affected by other variables and make the results more precise, the quantity on marble chips needs to be consistent. I will use a top pan balance for measuring the marble chips because it measures to the nearest 0.01g. It will be more accurate than measuring the marble chips by the number of spatulas I've put in because the results would vary.

I will use a burette to measure the quantity of hydrochloric acid and water because it would be more precise than using a measuring cylinder. This is because a burette measures to the nearest 0.1ml, where as a normal measuring cylinder only measures to the nearest ml. This would make the results more accurate.

I will use a gas syringe to measure the volume of gas because it is easier and more accurate than using a water trough. This is because a water trough might let some gases escape and it's hard to control the volume of water inside the measuring cylinder. This will make the results less accurate. Using a gas syringe will let the gas flow more easily and will not let any gases escape.

I will clamp the gas syringe so it's not tilted and is level with the conical flask. This way, the gas syringe will be less affected by gravity and the gas will flow at a steady rate. If it was not in place, gravity would pull the end down so the timing it takes for the volume of gas to reach 50ml would be altered and the results would not be right.

I will take assistance from someone to help me add the solution of acid and water into the conical flask with the marble chips, while I place the cork into the conical flask. This is because I can start the stop watch when the reaction starts quicker. If I were to do this alone, by the time I placed the solution and then start the stop watch, some of the reaction would have already started so the time it takes for the gas to reach 50ml would be incorrect.

I will keep the temperature the same each time because it has a significant effect on the rate of reaction. To make sure temperature does not affect the investigation, I will use a thermometer to measure the temperature of the acid and water because it will be the quickest. I have chosen the temperature of the acid and water to be 22&deg; because it was the temperature of the room at that time and the temperature is less likely to change this way. If the solution it higher or lower than this temperature, we will place the beaker of water and acid into a larger container of hot or cold water to bring the temperature to 22&deg;. This will be easier than using a Bunsen burner because that would change the temperature too dramatically.

From my preliminary results, I found measuring the time it takes for the gas to reach 50ml would be best. This is because every reaction reached 50ml with the different percentage of concentration used. Using measurements below 50ml would be harder because the time it takes to reach it will be too quick so, I will not be able to stop the stopwatch precisely on time. This would make the results more likely to be incorrect.

I will repeat this experiment thrice in order to get accurate results. This is because we can compare our results with two others and make an average. It will also make it easier to spot any outliers.

I have chosen to change the concentration of acid by 20ml each time because when I tried to change by 10ml, the time it took for the gas to reach 50ml was too long. From preliminary results, I found that by making the solution 100ml also takes too long or the reaction to occur, so now I will make the solution 200ml. Now, the rage of results will be less and will be easier to compare.

SAFETY

I will be using acid, so at all times I must be very careful and MUST wear goggles. If any spills onto the skin, it should be washed off immediately.

Other Methods

I could have carried out this experiment in different ways. Firstly I could react the marble chips with hydrochloric acid and using a top pan balance, and measure the weight of the reactants at the beginning of the reaction and measure the weight at the end of the reaction for a certain period of time and observe the quantity of gas given off using different concentration of acid.

The method is acceptable for reactions producing carbon dioxide or oxygen, but not very accurate for reactions giving hydrogen (too low a mass loss for accuracy). The reaction rate is expressed as the rate of loss in mass from the flask in e.g. g/min based on the initial gradient

Secondly, I could have used Sodium Thiosulphate and hydrochloric acid and see how the concentration of acid affected the time taken for the reactants to produce a cloudy precipitate. The experiment involves watching a black mark disappear through the cloudy sulphur and timing how long it takes to disappear.

How the concentration of acid is measured

The unit for concentration is moles per decimetre cubed or mol/dm³. One dm³ is one litre so in other words it is how many moles per litre.

A mole is 6.023 x 1023 atoms and is called the Avogadro constant. It is the number of atoms which are found in a mass of 12 grams of carbon-12 or 16 grams of oxygen-16 and so on. Therefore 2 moles of carbon-12 would be 24 grams of carbon-12 and 3 moles would be 36 grams of carbon-12 etc. The number of moles can be found by dividing the mass of the substance by its relative atomic mass (R.A.M.).

Example: How many moles of atoms would there be in 7 grams of nitrogen? The answer would be mass divided by the R.A.M. or 7 divided by 14 which is 0.5 moles.

Table showing how to dilute hydrochloric acid to the desired concentration

Concentration of acid (M)

Amount of 2M acid (ml)

Amount of water (ml)

0

0

200

0.1

10

190

0.2

20

180

0.3

30

170

0.4

40

160

0.5

50

150

0.6

60

140

0.7

70

130

0.8

80

120

0.9

90

110

1.0

100

100

Results

Generally, the graph shows the higher the concentration of acid, the less time it takes for gas to give off, which means, the faster it reacts. I drew a line of best fit which in fact proved to start fairly straight but then curve upwards. The gradient of the lines starts increasing slowly, then curves very steep. From 200ml-60ml of hydrochloric acid, the time increases from 1.13s-10.63. The time span increases by 9.5seconds. The average rate of reaction increased here by -5.2ml/s. From 60ml-20ml of hydrochloric acid, the time soared from 10.63s- 379.89. The time increased by 369.26s. The time increased by -123ml/s. Overall, the average rate of reaction was rate of reaction decreased by 37.9ml/s.

The reason why the line of best fit curves at the end is due to the fact that with the lower concentrations, as the HCl is used by reacting with the marble chips, the amount of H+ decreases and hence it is less likely that the H+ ion collide with the marble chips. This explains why the line is so much flatter at the beginning compared to with higher concentrations. If a lot larger quantities were used in the experiment, then we could have got over this problem and the curve would not show up.

The reason why the rate of reaction increases with the concentration is because when the concentration of the acid is increased, there are a greater number of H+(aq) ions in the same volume and hence there is a greater chance of the H+(aq) colliding and therefore reacting with the atoms on the surface of the marble chips. All the points on my graph are fairly close to the line of best fit and there are no anomalies, which in turn mean that my results are fairly accurate.

Evaluation:

To evaluate, I am going to assess how well I carried out my investigation and whether any improvements are necessary.

My method was fairly accurate. However, the fact that I used an analogue thermometer to measure the temperature of the hydrochloric acid in my method also means that I could have made an error in measuring the right temperature and it may have been better to use a digital thermometer. Additionally, I could have heated the water in a hot water bath to make sure that the water was at the right temperature.

One major flaw in the method I was using included the removal and application of the cork on to the conical flask containing the reaction. When I started the reaction by adding the calcium carbonate chips into the hydrochloric acid, I had to put the cork on straight away. However, this is not physically possible and it is inevitable that there will have been some time lost and even more importantly some gas would have escaped before I would have put the cork on. To work around this problem, I could separate the marble chips from the hydrochloric acid even in the conical flask by inserting a division between the two reactants. This will allow me the time to put the cork on and then I can remove the partition.

One more problem with my method is the time when I am supposed to take a reading. When I said to my partner when to stop, she might have stopped after 50ml gas was given off. To make it more precise, data logging equipment could have been used, which would have carried the same task out automatically.

However, I do believe that my experiment was conducted very well, as I had taken all the safety precautions (wore goggles, cleared the area etc.) and checked all of my equipment prior to carrying it out. After each time I had completed the method, I washed the required equipment thoroughly to ensure that no excess calcium carbonate was left over for the next time I carried out the method. My results are also reliable because I have got enough points (10) to put on my graphs that I can draw a firm conclusion from). Yet, I still obtained some anomalous results.

I found a few anomalies in my results, which are circled on my graphs on the previous pages. I believe these may have occurred due to a slight problem with the gas syringe, where the syringe may have been a bit stuck and thus resulted in a slower reaction time for when the gas is collected. Another reason for the anomalous points could include me taking incorrect readings, where the syringe may have been going too fast and to the naked eye, the reading may seem different to what it actually is.

I believe, to create a fairer and overall more reliable experiment, using the experiment where you mark a black cross under a conical flask and wait for precipitation to form and make the cross disappear would be a more appropriate technique. Also, I could measure the speed of this reaction by using a light gate to detect the precipitate formation. The system consists of a light beam emitter and sensor connected to computer and the reaction vessel is placed between the emitter and sensor. The light reading falls as the sulphur precipitate forms. This would be more accurate because it would decrease human error compared with the experiment I carried.